This invention relates to a surface measurement instrument for measuring a surface characteristic, in particular but not exclusively a surface measurement instrument such as a broad band or coherence scanning surface measurement instrument.
Coherence scanning or broadband scanning interferometry uses an interferometer (for example a Mirau, Michelson or Linnik interferometer) with a broadband spatially incoherent light source such as a quartz halogen lamp. Generally, but not necessarily, the broadband source will be a white light source. One of the sample surface whose surface characteristic is to be determined and the reference mirror of the interferometer is moved relative to the other along a scan path to change the relative path length and a two dimensional image sensor is used to sense the resulting interference pattern which changes as the sample surface and reference mirror are moved relative to one another.
Each sensing element or pixel of the image sensor senses the portion of the interference pattern for a corresponding portion (herein a “surface pixel”) of the sample surface and, as the sample surface and the reference mirror are moved relative to one another, the amount or intensity of light received by the sensing element will vary in accordance with the change in the interference fringes. The intensity of light received from a surface pixel of the sample surface will increase or decrease in amplitude in dependence upon the path length difference between the light paths from the reference mirror and the sensing surface and will have a coherence peak or extremum (maximum or minimum amplitude) at the position of zero path difference. Where different surface pixels of the surface have different relative heights, then those different surface pixels will have coherence peaks at different positions along the scan path. Accordingly, the relative positions of the coherence peaks can be used to provide surface characteristic data representing the relative heights of the different surface pixels of the sample surface.